Various types of exercise machines are known, and they provide adjustment to vary the resistance of the machine for the needs of the particular user, with it being recognized that some users are stronger than others.
Cost is an important item, because if the cost is too great the machines will not be employed.
When simple weights were used on bars, that progress of the person using the weighted bars for exercise could be measured very accurately, by the total weight applied to the bars for different exercises. However, with the advent of more sophisticated machines, there has developed a considerable problem in accurately determining the progress of a user, because with what appears to be the same set-up, the resistance will vary greatly from day to day due to internal friction and inaccuracies in the apparatus. That is, the repeatability for a desired result is quite poor.